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Optical changes that accompanied a collapse of the population of filamentous cyanobacteria from a shallow, eutrophic lake were studied in laboratory-scale enclosures (LSEs). The experimental conditions are known, from previous work on these systems, to cause a dramatic collapse of the dominant algal or cyanobacterial species, which in turn can be associated with viral activity. Within 2 weeks of continuous addition of nutrient-rich growth medium, near-complete collapse of the dominant population occurred over the span of a few days. The collapse was repeatedly and reproducibly observed and was primarily characterized by a marked increase in water transparency. Scattering of light decreased by 80%, absorption decreased by 20-80%. There was high similarity in optical changes between several experiments, carried out in different seasons. An increase of dissolved material and submicron-sized particles (SMP) that showed chlorophyll a (Chl a) absorption was observed during the collapse. The phycocyanin (PC): Chl a ratio and phaeopigment : Chl a ratio proved to be good indicators of the observed collapse. Reflectance spectra that were modelled using a constant volume-scattering function indicated that mass mortality of this magnitude can be detected in natural systems using current remote sensors.